1. Field of the Invention
The present invention relates to a method and apparatus for wire-bonding a first bonding point and a second bonding point.
2. Prior Art
Wire bonding method is performed, for example, as shown in FIGS. 5(a) through 5(g).
First, as shown in FIG. 5(a), a spark discharge of an electric torch 5 makes a ball 3a at the end of a wire 3 extending out of the lower end of a capillary 4. Then, the electric torch 5 moves away in the direction indicated by the arrow.
Next, as shown in FIG. 5(b), the capillary 4 is moved to a point above the first bonding point 1a and then lowered as shown in FIG. 5(c). The ball 3a at the end of the wire 3 is connected to the first bonding point 1a.
Afterward, as shown in FIG. 5(d), the capillary is raised and, as shown in FIG. 5(e), and moved to a point located above the second bonding point 2a. Then, the capillary 4 is lowered as shown in FIG. 5(f), and the wire 3 is bonded to the second bonding point 2a.
Next, the capillary 4 is raised to a predetermined position, a clamper 6 is closed, then the capillary 4 and the clamper 6 are raised together so that the wire 3 is cut as shown in FIG. 5(g).
With the steps described above and shown in FIGS. 5(a) through 5(f), one wire connection is completed.
Japanese Patent Application Laid-Open (Kokai) No. 57-87143 and Japanese Patent Application Publication (Kokoku) No. 1-26531, etc. are examples of this wire bonding method.
As seen from the above, in these prior art, the ball 3a is pressure-bonded (thus, the bonded ball 3a is called "pressure-bonded ball" hereinafter) to the first bonding point 1a; therefore, the relationship between the shape of the pressure-bonded ball and the bonding surface is a critical factor for a satisfactory joining or bonding.
In the prior art, as shown in FIG. 6, the capillary 4 is kept under a constant load 10, and an application load 11 is applied to a first bonding surface when the capillary 4 is in contact with the first bonding point. With this application load 11 being applied, an ultrasonic wave 12 is applied to the first bonding point.
In the prior art described above, however, no consideration is given to the application load 11 which affects the diameter of the pressure-bonded ball. The inventor of the present application conducted extended tests on the application load to find out the diameters of pressure-bonded balls which can provide a satisfactory joining (or bonding) strength.
The tests indicate that if the bonding surface application load 11 is large, the diameter of the pressure-bonded ball which has a satisfactory joining strength is large, but the differences in diameters of the pressure-bonded balls is small as shown in FIG. 7(a). On the other hand, if the application load 11 is small, the diameter of the pressure bonded ball which has a satisfactory joining strength is relatively small, but the differences in the pressure-bonded ball diameter is large as shown in FIG. 7(b).
Thus, if the pressure-bonded ball 3a is deformed by a large application load 11 before applying the ultrasonic waves, the pressure-bonded ball diameter can be consistent but the diameter becomes large, because the application load 11 during the application of the ultrasonic waves becomes to be excessive. On the other hand, if the application load 11 is small, the deformation of the pressure-bonded ball 3a before applying the ultrasonic waves is too small and not consistent in shape, but the diameter becomes small.
In other words, in the prior art, it is almost impossible to obtain an appropriate pressure bonded ball diameter and to minimize the differences of the pressure-bonded ball diameters.
In recent years, the semiconductor assembling devices have demanded a more and more precise and detailed work. As a result, it is very important to accomplish bonding with a minimal pressure-bonded ball and to reduce the differences or variation in the pressure-bonded ball diameter.